A sensitive promoter assay based on the transcriptional activator Tat of the HIV-1 virus

An attenuated herpes simplex virus type 1 (HSV1) encoding the HIV-1 Tat protein protects mice from a deadly mucosal HSV1 challenge

Herpes simplex virus types 1 and 2 (HSV1 and HSV2) are common infectious agents in both industrialized and developing countries. They cause recurrent asymptomatic and/or symptomatic infections, and life-threatening diseases and death in newborns and immunocompromised patients. Current treatment for HSV relies on antiviral medications, which can halt the symptomatic diseases but cannot prevent the shedding that occurs in asymptomatic patients or, consequently, the spread of the viruses. Therefore, prevention rather than treatment of HSV infections has long been an area of intense research, but thus far effective anti-HSV vaccines still remain elusive. One of the key hurdles to overcome in anti-HSV vaccine development is the identification and effective use of strategies that promote the emergence of Th1-type immune responses against a wide range of epitopes involved in the control of viral replication. Since the HIV1 Tat protein has several immunomodulatory activities and increases CTL recognition of dominant and subdominant epitopes of heterologous antigens, we generated and assayed a recombinant attenuated replication-competent HSV1 vector containing the tat gene (HSV1-Tat). In this proof-of-concept study we show that immunization with this vector conferred protection in 100% of mice challenged intravaginally with a lethal dose of wild-type HSV1. We demonstrate that the presence of Tat within the recombinant virus increased and broadened Th1-like and CTL responses against HSV-derived T-cell epitopes and elicited in most immunized mice detectable IgG responses. In sharp contrast, a similarly attenuated HSV1 recombinant vector without Tat (HSV1-LacZ), induced low and different T cell responses, no measurable antibody responses and did not protect mice against the wild-type HSV1 challenge. These findings strongly suggest that recombinant HSV1 vectors expressing Tat merit further investigation for their potential to prevent and/or contain HSV1 infection and dissemination.

Expression of human immunodeficiency virus type 1 tat from a replication-deficient herpes simplex type 1 vector induces antigen-specific T cell responses

Herpes simplex type-1 virus (HSV-1) based vectors have been widely used in different gene therapy approaches and also as experimental vaccines against HSV-1 infection. Recent advances in the HSV-1 technology do support the use of replication defective HSV-1 as vaccine vectors for delivery of foreign antigens. We have examined the ability of a recombinant replication-defective HSV-1 vector expressing the HIV-1 Tat protein to induce long-term Tat-specific immune responses in the Balb/c murine model. The results showed that vector administration by the subcutaneous route elicits anti-Tat specific T-cell mediated immune responses in mice characterized by the presence of the Tat-specific cytotoxic activity and production of high levels of IFN-gamma.

Reporter gene vectors and assays

Gene reporter systems play a key role in gene expression and regulation studies. This review describes the ideal reporter systems, including reporter expression vector design. It summarizes the many uses of genetic reporters and outlines the currently available and commonly used reporter systems. Each system is described in terms of the reporter gene, the protein it encodes, and the assays available for detecting presence of the reporter. In addition, each reporter system is analyzed in terms of its recommended uses, advantages, and limitations.

On the role of the second coding exon of the HIV-1 Tat protein in virus replication and MHC class I downregulation

Tat is an essential protein of human immunodeficiency virus type 1 (HIV-1) and activates transcription from the viral long terminal repeat (LTR) promoter. The tat gene is composed of two coding exons of which the first, corresponding to the N-terminal 72 amino acid residues, has been reported to be sufficient for its transcription function. We introduced a stop codon at the end of the first Tat-coding exon in an expression vector that produces a truncated 71-amino acid Tat protein. This Q72stop mutant displays reduced transcriptional activity of approximately 54% in transient LTR-CAT transfection assays. To test the contribution of the second Tat-coding exon to virus replication, the Q72stop mutation was also introduced in the infectious pLAI molecular clone. The effect on virus replication was analyzed in primary cells and in a transformed T cell line. The fitness of the mutant virus was calculated to be approximately 75% compared with the wild-type control. Thus, a small contribution of the C-terminal Tat domain to viral fitness was measured. It has been proposed that the second Tat-coding exon is involved in transcriptional downregulation of the MHC class I gene of the infected host cell. Cell surface expression of the MHC protein was analyzed in T cells infected with the wild-type LAI virus and the replication-competent Q72stop mutant. MHC expression was transiently reduced on infection with either virus, indicating that the second Tat-coding exon is not involved in this downregulation.

Determination of the minimal amount of Tat activity required for human immunodeficiency virus type 1 replication

The Tat protein of human immunodeficiency virus type 1 (HIV-1) is a potent trans-activator of transcription from the viral LTR promoter. Previous mutagenesis studies have identified domains within Tat responsible for binding to its TAR RNA target and for transcriptional activation. The minimal Tat activation domain is composed of the N-terminal 48 residues, and mutational analyses identified a cluster of critical cysteines. The importance of four highly conserved aromatic amino acids within the activation domain has not been thoroughly investigated. We have systematically substituted these aromatic residues (Y26, F32, F38, Y47) of the HIV-1 LAI Tat protein with other aromatic residues (conservative mutation) or alanine (nonconservative mutation). The activity of the mutant Tat constructs was measured in different cell lines by transfection with a LTR-CAT reporter plasmid. The range of transcriptional activities measured for this set of Tat mutants allowed careful assessment of the level of Tat activity required for optimal viral replication. To test this, the mutant Tat genes were introduced into the pLAI infectious molecular clone and tested for their effect on virus replication in a T-cell line. We found that a twofold reduction in Tat activity already affects viral replication, and no virus replication was measured for Tat mutants with less than 15% activity. This strict correlation between Tat activity and viral replication demonstrates the importance of the Tat function to viral fitness. Interestingly, a less pronounced replication defect was observed in primary cell types. This finding may correlate with the frequent detection of proviruses with Tat-inactivating mutations in clinical samples.

Inhibition of human immunodeficiency virus expression by sense transcripts encoding the retroviral leader RNA

Towards gene therapy for the treatment of human immunodeficiency virus type 1 (HIV-1) infections, we tested the potency of several antiviral constructs in transient HIV-1 production assays. Whereas little effect was obtained with antisense- and TAR decoy-constructs, we measured efficient inhibition of HIV-1 mRNA translation and virion production in the presence of HIV-1 leader-containing transcripts. The infectivity of these virions was also reduced by this sense inhibitor RNA. These results suggest that leader-encoded functions, like the dimer-linkage structure, can be used to specifically inhibit HIV expression in trans.